1. Field of the Invention
This invention is concerned with systems and methods of forming images on photosensitive printing plates used in printing press systems.
2. Description of the Related Art
Modem printing presses, such as those used to print newspapers and magazines, typically employ photosensitive printing plates. A desired image is formed on the printing plate, which is then installed in a press and repeatedly used to produce copies of the formed image.
Several methods are used to form images on the photosensitive plates. For example, a negative film containing a desired image can be placed over the printing plate. Light to which the plate is sensitive (typically ultraviolet (UV);) is directed through the film onto the plate, exposing the plate and thereby transferring the desired image. However, preparation of a negative film for each image can be a time-consuming and costly process.
The use of a negative film can be avoided by using a xe2x80x9ccomputer-to-platexe2x80x9d (CTP) process, in which a laser is used to expose the plate directly. Here, a laser beam exposes one pixel at a time as it is deflected across the plate. The exposure time of the plate, along with the time allotted to expose each pixel, define the power requirement of the laser, which must have sufficient power to expose the plate in the allotted time.
Photosensitive printing plates are typically sensitive to UV light, and a CTP process using such a plate would require a UV laser. However, UV lasers capable of providing the required power tend to be large, and have special reliability and cooling requirements, making them prohibitively expensive. Printing plates are available that are sensitive to more affordable red or green lasers and which require much less power to expose, but these plates also tend to be very expensive.
A need exists for a method of exposing affordable UV-sensitive printing plates via a CTP process that does not require the use of expensive UV lasers.
A platemaking system and method are presented which overcome the problems noted above, enabling low-cost printing plates to be exposed with inexpensive light sources in a CTP process.
The novel process makes use of an intermediate imaging mask made of a film of photochromic material, preferably bacteriorhodopsin. The photochromic film is nominally substantially transparent, but is temporarily made substantially opaque by exposure to light of a first wavelength. A desired image is formed on the mask, preferably by deflecting a laser beam operating at the first wavelength across the mask, which renders portions of the mask opaque. The mask is then interposed between a photosensitive printing plate and a light source which produces light to which the plate is sensitive, exposing the printing plate and thereby transferring the desired image onto the plate.
The photochromic film is preferably made from bacteriorhodopsin (bR). Bacteriorhodopsin is initially transparent to UV/violet light, but becomes opaque to UV/violet light when exposed to green light (suitably provided with an inexpensive green or yellow laser). Those portions of the bR-based film which have been transformed remain opaque to UV/violet light for up to a minute or more, enabling the film to serve as an intermediate imaging mask and eliminating the need to employ special high power equipment that might otherwise be required in a CTP system. Once the image is formed on the bR-based mask, a UV/violet (non-laser) light source can expose conventional low-cost UV-sensitive printing plates through the mask to transfer the desired image onto the plate.
The described method is preferably implemented using a transparent rotating drum that is coated with a bR film. As the drum rotates past a green or yellow laser, a desired image is written onto the film by the laser""s beam. A printing plate is placed adjacent to the surface of the drum; the drum rotates the image until it is opposite the plate, and a light inside the drum exposes the plate through the mask to transfer the image.
In the absence of green light, bR eventually reverts to its transparent (to UV/violet light) state naturally, though the reversion can be delayed by exposing the mask to both green/yellow and UV/violet light simultaneously. By exposing the bR film to UV/violet light, this transformation is made practically instantaneous. Therefore, a platemaking system per the present invention which uses a bR-based mask preferably includes a UV/violet light source, which is directed onto the bR film after the image has been transferred to xe2x80x9cerasexe2x80x9d the film and prepare it for a new image.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.